Sensor devices for measuring at least one property of a fluid medium, preferably of a gas, are available. These include sensor devices having at least one sensor element for measuring at least one parameter of a gas, in particular at least one property of an exhaust gas of a combustion engine, such as the concentration of a constituent of the exhaust gas, in particular the oxygen concentration, the nitrogen oxide concentration, and/or the concentration of gaseous hydrocarbons, for example. Other properties that can be measured using such a sensor device are the particulate matter formation, the temperature and/or the pressure of the fluid medium, for example. In particular, such a sensor device can be a lambda probe. Lambda probes are often used in the exhaust branch of an internal combustion engine, above all for measuring the partial pressure of oxygen in the exhaust gas. Lambda probes are described, for example, in Konrad Reif, Sensoren im Kraftfahrzeug (Sensors in the Motor Vehicle), Springer Wieweg publishers, 2nd edition, 2012, pp. 160-165.
At the exhaust gas-side tip thereof, such sensor devices notably have a protective housing that extends into the exhaust stream. The protective housings are used for protection against mechanical stresses that arise during installation, as well as due to particles that occur in the exhaust system, and are used for a controlled guidance of the flow of the fluid medium within the sensor device to the sensor element located therein, as well as for protecting the sensor element from a condensate from the exhaust gas and from an attendant thermal shock to the sensor element. What is commonly known as thermal shock occurs, in particular, when a condensate drop forms from the exhaust stream and precipitates onto the hot, ceramic sensor element, thereby producing local temperature differences on the surface of the sensor element that can result in high, thermally induced stresses in the sensor element that can eventually lead to damage to or even destruction of the sensor element. The protective housing is normally designed to minimize a loading of the sensor device with liquid occurring in the exhaust system, preferably to a volume that is harmless to the sensor element, to a dew point temperature. Moreover, to protect the sensor element from thermal shock, it is preferably also provided with a coating for purposes of thermal insulation and/or fluid binding. In this connection, it is especially advantageous that the coating include a ceramic, in particular an aluminum oxide.
In many cases, however, the requirements for the protective housing are contradictory. In practice, there is, in particular, a conflict of objectives between the requirements for a high level of protection from thermal shock and for a high dynamic response of the sensor device. This especially means that measures performed on the protective housing, that lessen the loading of the sensor element with liquid, often simultaneously reduce the dynamic response of the sensor device. This is due to the fact that, normally, a most rapid possible gas exchange near the sensor element enhances the dynamic response of the sensor device, while the liquid loading of the sensor element is simultaneously hereby increased, generally thereby reducing the protection against thermal shock. In practice, this means that, normally, only one of the two requirements, high dynamic response or high level of protection against thermal shock, can largely be met satisfactorily for a certain selected protective housing.
The protective housing itself may have a one-part or multipart design, a multipart design mostly having an inner housing and an outer housing surrounding the inner housing, between which an intermediate space is formed, in which further protective tubes are possibly located. Protective housings having two or three protective tubes and which, therefore, are also referred to as double or triple protective tube (housings), are used very frequently. A triple protective tube normally provides a better protection against thermal shock than does a double protective tube. However, there are a number of disadvantages inherent in the conventional triple protective tubes. In comparison with a double protective tube, a triple protective tube normally has a longer flow path that the gas stream must travel from the inlet openings at the protective housing to the sensor element. Such a design is, in fact, conducive to protection against thermal shock, but mostly degrades the dynamic response of the sensor device due to the longer flow path. Because of the more stringent requirements placed on the dynamic response, conventional triple protective tubes are no longer suited for such purposes. Moreover, the triple protective tube design, which is customary at present, requires that the inner housing be introduced from the side of a reference air space into the outer housing. Since the diameter of the inner housing must be limited as a result, the limited space in the inner housing necessitates using only those sensor elements that do not have any additional protective coating against thermal shock. However, the lack of a sensor element coating considerably diminishes the liquid volume from the exhaust stream that the sensor element can act upon. In spite of an additional protective tube, this usual triple protective tube configuration ultimately results in the protective action of the entire sensor device against thermal shock being altogether only insignificantly or not enhanced since the sensor element's protection against thermal shock is reduced.
European Patent Application EP 2 333 534 A2 describes a sensor device for measuring at least one property of a fluid medium that is provided with a protective housing for accommodating at least one sensor element. Located within the protective housing is the flow path that is traversable by the fluid medium flow and that extends from the inlet openings in the outer housing across the intermediate space and access openings in the inner housing to the sensor element; within the protective housing, the flow path having two deflection points where the fluid medium undergoes a directional change by an angle of 90°. In addition, at least one wall body is provided within the protective housing along the flow path. It is configured and adapted to absorb heat from the fluid medium that is moved past the wall body at a lowest possible velocity.